The present invention relates to an erbium (Er)-doped optical fiber laser device.
In optical communications there has heretofore been employed, for amplifying an optical signal of reduced intensity, a method which once converts the optical signal into an electric signal through a photodetector, amplifies the electric signal and then converts the amplified electric signal into a high-intensity optical signal by use of a semiconductor laser.
On the other hand, there has recently been proposed a so-called direct optical amplification (hereinafter referred to as "optical amplification") system which amplifies an optical signal as it is. Much study is being given for this system in many countries as an indispensable technique for future optical communications and photometry, because the system does not involve the photo-electrophoto conversion and hence permits an arbitrary selection or change of the bit rate and scimitareous amplification of wavelength multiplex and frequency multiplex optical signals.
There has been heretofore proposed an optical amplification method which employs, as an optical amplifier, optical fiber doped with a rare earth element. It has been ascertained that an optical amplifier capable of amplifying signal light of a 1.5 .mu.m band in the range of 20 to 30 dB can be obtained by doping silica glass fiber with erbium (Er) which is one of rare earth elements (which amplifier will hereinafter referred to as an "Er-doped optical fiber laser amplifier"). Because of its various advantages such as a high saturated output level, low polarization dependence and a small noise figure, this optical amplifier is now being actively investigated as one of practical optical amplifiers in the future.
However, the conventional Er-dope optical fiber laser device for amplification or oscillation has defects of large size, low efficiency and low reliability because of a large difference between the oscillation (or amplification) wavelength in the 1.5 .mu.m band and the wavelength of the pumping light source in the range of 0.5 to 1 .mu.m.